Due to the limited bandwidth of transmission channels, there are a limited number of bits available for encoding image information, such as image information generated by a camera for transmission to one or more remote users. Thus, there are many image encoding techniques available which encode the image information with as few bits as possible using compression techniques, while still maintaining the quality and intelligibility that are required for a given application.
Remote cameras, such as those used for security applications, traffic monitoring or daycare monitoring, are typically panned by physically moving the camera. In addition to the possibility of a mechanical failure, the utility of such remote cameras is limited in that only one user can control the camera at a time. For multi-user applications, however, such limited user control of the camera view is not practical. A number of software techniques have been developed for permitting a number of users to view selected portions of a larger image (or a composite image generated from a plurality of individual images).
Permitting multiple selected views of a larger image, however, becomes more difficult if the larger image is compressed. Specifically, since image data following image compression is of variable length, pixel boundaries are not readily detectable in a compressed image. In addition, since many encoding techniques exhibit intra-frame pixel dependencies, such as encoding the difference values for adjacent DC coefficients under the JPEG standard, the pixel values must be modified when generating a selected portion of a larger image, to reflect the reordering of the subset of pixels in the selected image view.
Typically, when generating a selected portion of a larger compressed image, the larger image must be decompressed into the pixel domain, before the pixel values are reordered and assembled to create each of the selected image views. Thereafter, each of the selected image views is compressed to form the final images transmitted to each user. The more popular image compression techniques, such as JPEG and MPEG, typically perform three steps to generate a compressed image, namely, (i) transformation, such as a discrete cosine transform (DCT); (ii) quantization; and (iii) run-length encoding (RLE). Likewise, to decompress images using these same image compression techniques, the inverse of the compression steps are performed by the receiver on the compressed image, namely, (i) run-length decode; (ii) dequantization; and (iii) inverse discrete cosine transform (IDCT).
Thus, to create N selected image views from a larger compressed image, conventional techniques require one image decompression, N pixel reorderings, and N compressions.